1. Field of the Invention
The invention relates generally to optical imaging using optical coherence tomography and in particular to systems and methods for achieving balanced detection with high signal to noise ratio.
2. Description of Related Art
Optical coherence domain reflectometry (OCDR) is a technique initially developed to provide a higher resolution over optical time domain reflectometry (OTDR) for the characterization of the position and the magnitude of reflection sites in such optical assemblies as optical fiber based systems, miniature optical components and integrated optics (Youngquist et al., “Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique”, 1987, Optics Letters 12(3):158-160). With the addition of transverse scanning, this technique has been widely and successfully extended to the imaging of biological tissues, and is termed optical coherence tomography (OCT) (Huang, D., E. A. Swanson, et al. (1991). “Optical coherence tomography.” Science 254 (5035): 1178-81; and U.S. Pat. Nos. 5,321,501 and 5,459,570). Since then, a large of number of applications have been found for this technology as evidenced by a number of review articles (Swanson E. A. et al. “Optical coherence tomography, Principles, instrumentation, and biological applications” Biomedical Optical Instrumentation and Laser-Assisted Biotechnology, A. M. Verga Scheggi et al. (eds.) pages: 291-303, 1996 Kluwer Academic Publishers, Printed in the Netherlands; Schmitt, J. M. “Optical coherence tomography (OCT): a review” IEEE Journal of Selected Topics in Quantum Electronics, Volume: 5, Issue: 4, Year: July/August 1999 pages: 1205-1215; Fujimoto, J. G. et al. “Optical Coherence Tomography: An Emerging Technology for Biomedical Imaging and Optical Biopsy” Neoplasia (2000) 2, 9-25; Rollins A. M. et al. “Emerging Clinical Applications of Optical Coherence Tomography” Optics and Photonics News, Volume 13, Issue 4, 36-41, April 2002; Fujimoto, J. G. “Optical coherence tomography for ultrahigh resolution in vivo imaging.” Nat Biotechnol 21(11): 1361-7, (2003)).
The U.S. patents cited above as well as those cited throughout this patent application are incorporated herein by reference.
The traditional interferometer configuration for OCDR or OCT is a standard Michelson interferometer. As shown in FIG. 1, light from a broadband, or frequency-tunable, source 10 is input into a beam splitter or 2×2 fiber optic coupler 12, where the light is split and directed into a sample arm 14 and a reference arm 16. An optical fiber 18 in the sample arm 14 extends into a device 20 that scans an object 22 with a beam of light. The reference arm 16 provides a variable optical delay. Light input into the reference arm 16 is reflected back by a reference mirror 24. A piezoelectric modulator 26 may be included in the reference arm 16 with a fixed reference mirror 24, or the modulator 26 may be eliminated by scanning the mirror 24 in the Z-direction. The reflected reference beam from reference arm 16 and the scattered sample beam from sample arm 14 pass back through the coupler 12 to detector 28 (including processing electronics), which processes the signals by techniques known in the art to produce a backscatter profile or image on a display unit 30.
This configuration is advantageous in that it uses a minimum number of optical components and is hence the simplest. It can be implemented using bulk or fiber optics or a combination thereof. However, this configuration is limited to an optical efficiency of 25% as explained below.
By examining the configuration, it is not difficult to discover that the optical power reaching the detector from the two arms is reciprocal with respect to the beam splitter or fiber coupler (BS/FC). Assuming that the power split ratio of the beam splitter is
  α      1    -    α  and neglecting loss in the splitter, the attenuation by the beam splitter or the fiber coupler (BS/FC) to both the sample optical wave and the reference optical wave is the same and is equal to α(1−α). The only difference is that for one wave it will propagate straight-through the BS/FC first with an attenuation caused by a transmission factor of (1−α) and then crossover the BS/FC with a further attenuation by a factor of α, whereas for the other wave, it will crossover the BS/FC first with an attenuation by a factor of α and then propagate straight-through the BS/FC with a further attenuation by a factor of (1−α). It is well known that for such a configuration, the most efficient power splitting ratio is 50/50, where
            α              1        -        α              =    1    ,simply because the function α(1−α) has its maximum value when α=0.5.
Due to the fact that the optical power reflected or scattered back from a biological sample is generally a few orders of magnitude less than the incident power, the reference arm optical power needs to be attenuated by two or more orders of magnitude to improve the signal to noise ratio [Sorin, W. V. et al. (1992) “A simple intensity noise reduction technique for optical low-coherence reflectometry.” Photonics Technology Letters IEEE 4(12): 1404-1406]. Otherwise, the signal will be degraded by the optical power noise of the source. Hence virtually 75% of the optical power supplied by the light source is wasted in this configuration.
Another issue with the classic Michelson interferometer (FIG. 1) is that light from the reference arm is coupled back into the optical source, causing side effects that can impact the quality of the resulting image.
In order to improve the optical power efficiency, use of circulator(s) in the sample arm, the reference arm and also in the source arm was disclosed in U.S. Pat. Nos. 6,134,003, 5,956,355, 6,175,669, 6,384,915 and in B. E. Bouma and G. J. Tearney (1999). “Power-efficient nonreciprocal interferometer and linear-scanning fiber-optic catheter for optical coherence tomography.” Optics Letters 24(8): 531-533. A summary was given by Rollins and Izatt [U.S. Pat. No. 6,657,727; A. M. Rollins and J. A. Izatt “Optimal interferometer designs for optical coherence tomography” Optics Letters, Vol. 24 Issue 21 Page 1484 (1999)]. As shown in FIG. 2, a key optical element that is used in these configurations is an optical circulator and such a circulator can be combined with unbalanced couplers, and (or) balanced heterodyne detection for optical power efficient interferometer construction. FIG. 2 encompasses 6 configurations where the three insets basically show a modification from the three corresponding balanced heterodyne detection employing balanced couplers to a single detector based detection employing unbalanced coupler(s). The first two configurations (Ai and Aii) are based on a Mach-Zehnder interferometer with the sample located in a sample arm and the reference optical delay line (ODL) in the other reference arm. The main difference between Ai and the standard Mach-Zehnder interferometer (MZ-interferometer) is that the prior fiber coupler, i.e. the fiber coupler between source and sample, has an optical power split ratio of
      α    1        1    -          α      1      instead of 50/50. This ratio can be optimized for optical power efficient high SNR detection by directing most of the original optical power to the sample arm. Meanwhile light is coupled to the sample through an optical circulator such that the backscattered optical signal is redirected to a different path containing a “post” fiber coupler. The reference arm ODL may be either transmissive or retroreflective (see U.S. Pat. No. 6,657,727). Note that in Ai, the post fiber coupler has a split ratio of 50/50 and due to the employment of balanced heterodyne detection, the SNR of Ai can be improved over that of a standard Michelson configuration as shown in FIG. 1 by 8 dB. However, due to the fact that a practical fast-scanning optical delay line operates in reflection mode, a second circulator is needed, and this will increase the cost of the system.
In configuration Aii, the post fiber coupler is also made unbalanced and a single detector is used. By properly controlling the optical power split ratio of both the prior fiber coupler and the post fiber coupler, a theoretical SNR improvement of 2 dB over the standard Michelson interferometer of FIG. 1 can be achieved. The advantage of Aii as compared to Ai is that since only one detector is used, the cost of the system will be lower than that of Ai.
Refer now to Bi and Bii, while the sample arm part is the same as in Ai and Aii, the reference arm ODL is made retroreflective. Note, to use a retroreflective ODL instead of a transmissive ODL in Ai and Aii would require a second circulator. Again, the optical power split ratio of both the prior fiber coupler and the post fiber coupler,
      α    1        1    -          α      1      and
            α      2              1      -              α        2              ,can be properly chosen for either the two detector based balanced heterodyne detection case or the unbalanced single detector case to optimize the SNR such that the system is optical power efficient. Izatt and Rollin showed that the SNR improvement of Bi and Bii is very similar to that of Ai and Aii. Note that there will be a small portion of the optical power from the reference ODL being returned to the light source path.
Configuration Ci and Cii are variations on the standard Michelson interferometer. Their difference as compared to FIG. 1 is the use of an optical circulator in between the light source and the fiber coupler to channel the returned light from the fiber coupler to the detector, d2. While for balanced heterodyne detection (Ci), the optical power split ratio of the fiber coupler must be made 50/50, it should be noted that for the case of single-detector unbalanced detection (Cii), the optical power delivered to detector d2 from the sample arm and the power from the reference arm can be made different. The sample optical signal will propagate straight through the fiber coupler twice and the reference optical signal will cross over the fiber coupler twice. As a result, the optical power delivery to detector d2 can be made efficient by properly selecting the fiber coupler optical power split ratio
      α          1      -      α        .Izatt and Rollin stated that for Ci, the SNR can be improved over that of FIG. 1 and although this configuration is not as power efficient as the other two, i.e. Ai and Bi, its significant advantage is that it can be easily retrofitted with a circulator in the source arm and with a balanced receiver, with no need to disturb the rest of the system. As for Cii, the SNR improvement is similar to that of Aii and Bii.
It can be seen from the above-mentioned configurations that the key advantage of these prior art configurations lies in the improvement of the optical power delivery efficiency to the detector(s), by properly selecting an optical power split ratio
  α      1    -    α  (for either the prior and/or the post fiber coupler). However, in the case of Ai and Aii, although the SNR is maximized, if a retroreflective ODL is desired in these configurations, a second circulator would be necessary which will increase the cost of the system. With the configuration of Bi and Bii, there will be a certain amount of light being channeled back to the source. At the same time, for Ai, Aii, Bi and Bii, there are always two 1×2 or 2×2 directional couplers used and if one needs to monitor the source power, another 1×2 or 2×2 tap coupler needs to be added to the system. The system hence costs more and is not very compact. For the cases of Ci and Cii, although the system is compact as there is only one 2×2 coupler, the optical power efficiency is not fully maximized as the power splitting ratio of the coupler must be made 50/50 for returned dual balanced detection, which will also force the forward split ratio to be 50/50. As a result, attenuation in the reference arm is needed to achieve shot noise limited detection and this will waste optical power.
Attempts have been made to use polarization dependent component(s) and/or coupler(s) to unevenly split the forward propagating optical power and evenly split the backward propagating optical waves. But these schemes generally require a complicated polarization dependent splitter design and are not very suitable for biological samples with varying birefringence [EP1253398, US Patent Application No.: 2005/0213103].
In this invention, a single monolithic 3×3 coupler is combined with a circulator in a looped sample path to realize almost all the advantages of a polarization independent high power efficiency and high SNR scheme. This configuration makes the system more compact and less costly. In addition, the fourth port of the circulator can be used for optical power monitoring of the source, which can further save the space and cost of a tap coupler. Alternatively, such a 3×3 coupler can also be mimicked by cascading two 2×2 couplers, which offers almost the same SNR advantage as that of the monolithic 3×3 coupler design.
It should be noted that although there are reports on using 3×3 couplers for optical coherence interferometry, in these prior art systems, the 3×3 coupler has an equal power split ratio for the three ports and the use of such a symmetric 3×3 coupler is meant for achieving simultaneous 3-phase detection rather than for optimizing optical power efficiency. [M. A. Choma, et al. (2003). “Instantaneous quadrature low-coherence interferometry with 3×3 fiber-optic couplers.” Optics Letters 28(22): 2162; DE 4403929A1; M. C. Tomic, et al. “Low-coherence interferometric method for measurement of displacement based on a 3×3 fibre-optic directional coupler” J. Opt. A: Pure Appl. Opt. 4 (2002) S381-S386; M. V. Sarunic, et al. “Instantaneous complex spectral domain OCT using 3×3 fiber couplers” SPIE, Vol. 5316, p241-247]